The 1971 Worldcon, Noreascon I was attended by 1600 people and was the largest Worldcon to date. Three years later, Discon II was attended by over 3500 people and fandom worried (with good reason) that Worldcon was growing beyond the ability of fandom to run it and into something which fandom no longer recognized.

(It's also worth remembering that Worldcon had not yet started to use a convention center along with a hotel, so Worldcon size was limited by what a single hotel could support.)

The root cause was probably the success of Star Trek in the late 60s which was very popular with existing fans and which brought many new people into fandom. Simultaneously, existing regionals were growing and new ones being founded.

The sheer growth in numbers caused consternation, but the development of sub-fandoms also worried many. For example, the SCA, founded in 1966, was well on the way to turning into a group separate from fandom, but with many members in common, and the Star Trek Conventions were busy growing into giants (the first, in 1972, in New York City expected 500 members (a very respectable size for a regional) and got 3000.)